The present invention relates generally to optical cables. Specifically, the present invention relates to filling compounds for light waveguide leads and/or light waveguide cables that includes a thixotroping agent.
Typically, several light waveguides leads, for the transmission of optical signals or light waves, are disposed within a protective casing defining a light waveguide cable. Each light waveguide lead typically consists of a fiber that is coated and surrounded by a protective sheath. As disclosed in U.S. Pat. No. 4,331,379, the space between the protective casing and light waveguide lead can be filled with a filling compound. U.S. Pat. No. 4,331,379 discloses a filling compound that consist of thixotropic oils. Moreover, the space between the fibers of the light waveguide lead and the protective sheath can also be filled with a filling compound.
Prior filling compounds have not been entirely satisfactory. This is due, in part, to the fact that the requirements of the filling compounds are, in part, contradictory for light waveguide transmission elements. It is desireable for a filling compound to exhibit the following properties:
(a) The filling compound should be resistant to aging both in a chemical sense and in a physical sense,
(b) The filling compound should be compatible with and not exhibit a detrimental effect on any of the materials the filling compound will contact, i.e. the protective casing, the coating on the fibers, or the protective sheath.
(c) The filling compound should exhibit an optimally low moisture absorption,
(d) The filling compound must retain its properties over a wide temperature range, for example from -40.degree. C. to +70.degree. C.; accordingly the filling compound should not exhibit excessive hardness at low temperatures that will cause a mechanical stressing of the light waveguides due to an excessive increase in the viscosity nor should the filling compound exhibit a phase separation at high temperature and resultant drip-out of the filling compound from the cable,
(e) the shearing strain characteristic of the filling compound is also important, shearing rate characteristic should be established without noticeable gradient, i.e. without excessively pronounced thixotropic behaviour; this is important for uniform lateral displacement of the light waveguide fibers given relative length changes between the protective sheath and the fiber,
(f) the filling compound should avoid the build-up of tensile or compressive forces on the light waveguide leads to the greatest possible degree, the increase tensile on compressive force will cause the light waveguide leads to attenuate,
(g) the filling compound should have a low viscosity at room temperature in its inprocessed condition in order to enable the filling of the cables and leads with the filling compound during the light waveguide manufacturing operation; even with cables and leads having a small inside diameter and at a slight excess pressure, and
(h) the filling compound should have a transparency that is such that it allows for the filling compound to have a thickness of up to a few millimeters.
Typical known filling compounds are defficient in one or more of the above characteristics. Thus, there is a need for an improved filling compound for light waveguide leads and light waveguide cables.